An uncalibrated index of oxygen saturation in blood can be determined using an implantable optical sensor measuring the attenuation of two light wavelengths, typically red and infrared. This oxygen saturation index (O2 index) is useful for monitoring short-term, relative changes in oxygen saturation to detect a physiological event or condition causing a relatively sudden or abrupt change in oxygen saturation. For example, the O2 Index can be used in detecting conditions or events affecting the oxygenation or hemodynamic status of a patient when changes occur over a relatively short period of time, e.g. within about 10 seconds, such as during an unstable cardiac arrhythmia.
The O2 is computed as a function of the normalized change in the attenuation of red light with respect to a baseline red light attenuation measurement and the normalized change in the attenuation of infrared light with respect to a baseline infrared light attenuation measurement. Depending on the selected value of coefficients applied to the red and infrared light terms used to compute the O2, the index can be influenced by both blood oxygen saturation and total hemoglobin concentration present in a measurement volume of the optical sensor. Other influences can sometimes cause the O2 to respond to oxygen saturation and hemoglobin concentration changes in unpredictable ways over relatively longer periods of time, limiting the usefulness of the O2 in longer-term monitoring applications.
A calibrated absolute oxygen saturation (StO2) can be obtained using a four wavelength optical sensor. Emission and measurement of four wavelengths however requires higher power and processing burden than a two wavelength measurement. A need remains for a device and method capable of efficient and reliable monitoring of oxygen saturation and hemoglobin concentration useful for both acute and chronic patient monitoring, including ambulatory patient monitoring.